Nobel Peace Prize laureate Adolfo Perez Esquivel yesterday announced that he and 11 other people would begin a prayer fast here Thursday to express their concern over the violence in El Salvador and generate ideas on ways to end the conflict.

Perez Esquivel, 50, who won the 1980 award for human rights work in his home country of Argentina, was among more than 1,000 people who gathered in the New York Avenue Presbyterian Church downtown yesterday for an ecumenical prayer service to commemorate the assassination two years ago of Salvadoran bishop Oscar Romero.

Entertainers Joan Baez and Jackson Browne provided music for the service, which was organized by the Religous Task Force on El Salvador, a coalition of religious groups. Rep. Tom Harkin (D-Iowa) and the Rev. Robert Drinan, a former congressman now president of Americans for Democratic Action, also were among the participants who gathered in the church and then walked to Lafayette Park for an outdoor rally.

At the rally, Harkin said protesters had gathered again "because our leaders have failed us." He said he wanted to tell President Reagan: "No, no more guns to El Salvador."

The service and rally came one day after a much larger demonstration, in which an estimated 23,000 persons gathered here to protest U.S. policy in El Salvador. Yesterday's activities also coincided with the elections in El Salvador.

Romero, who had been a vocal critic of his government and who had urged the U.S. not to send any military aid to his country, was killed as he said mass on March 24, l980. He has become a symbol for many Americans who oppose the administration's involvement in the Salvadoran war.

"We fear that our own government's policy of ever-increasing military aid to the current regime is causing prolonged death and destruction," said Jack Malinowski, spokesman for the coalition.

"We urge our government to renounce a military solution in El Salvador and to commit U.S. power and prestige firmly behind a process of negotiated political settlement," he said. He also emphasized the action is a fast and not a hunger strike, which he said usually has a specific political goal.

In the church, hung with a huge picture of Romero, Baez sung for the first time a new ballad called "El Salvador" which praises "los campaneros," or peasants, "born in the war from Belfast to San Salvador," and "from Warsaw to San Salvador."